Make apex availability of framework-wifi-util-lib explicit

The availability to wifi apex was done implicitly using a baseline map in
build/soong/apex/apex.go. Make this explicit in Android.bp

Bug: 281077552
Test: m nothing
Change-Id: I5c3fb582f290da2d5cedc65417ca126013941da5
diff --git a/Android.bp b/Android.bp
index ebdb1a1..2113e4b 100644
--- a/Android.bp
+++ b/Android.bp
@@ -647,6 +647,7 @@
     lint: {
         baseline_filename: "lint-baseline.xml",
     },
+    apex_available: ["com.android.wifi"],
 }
 
 filegroup {